There are two factors at play when dealing with long cable runs. the first factor is power to the LNB. using the 29v should be OK and you should have no issues. the second factor is RF, the 29v power supply wont help you there, this is where RG11 might come into place. But @ 250 feet, I think you should be OK with RG6 as long as you dont over do it with the splitters

Should I take out the inline amp I just installed when I install the 29V PI?

My signal strength is only below 85 on a few of the 99 degree satellite transponders (74,76,77,78 on four transponders) all the 101 and 103 satellite transponders are above 85. Do you think RG11 would help on these signal strengths or is it not worth the effort?